Add timer (count down in seconds) as secondary line

1

I have mushroom card:

type: custom:mushroom-template-card
entity: heater_entity
icon: mdi:radiator
icon_color: |
  {% if is_state('heater_entity', 'on') %}
    green
  {% else %}
    gray
  {% endif %}
primary: heater
tap_action:
  action: toggle
secondary: ''

how can I add as a second line time count down in seconds?
in automation YAML I have define 600 seconds
00:10:00
00:09:59
00:09:58
etc…

Thanks